Thermal storage & controls

[Bob Rohr]

standard domestic water heaters use stratification in a similar way. As the hot water is drawn from the tank, the cold water fills in from the bottom up. this gives hot water at the outlet for much longer. If the water in the tank was mixed, the outlet water temperature would start to drop right away as the cold water averages with the hot water.

In a similar way a large storage tank with a vertical heat exchanger coil will be able to deliver, say, 130 degree water even though the bottom half (?) of the tank is well below 130. basically it allows a better drawdown and increased delta t of the storage.

Another benefit is that if there is a layer of cold water on the bottom of the tank, there will be less heat loss from the bottom because the temperature difference is less.

They tell me a typical 50 gallon water heater set at 130F with 55F incoming water will only proveid about 38-40 gallons of hot or warm water due to the mixing. I wonder hw hot the top of the tank is if the aquastat trips off at 130?

These days I see most homes with DHW recirc pumps. I'd guess the tank would be blended well at 130F?

Now if the tank was blended at 180F it seems there are more btu then in a tank with a stratification of cool water at the bottom.

I need to ring up some folks in the water heater industry to get cler on the math to determine which would store more energy. if the boiler or water heater is setpoint-ed at 130 or 180 for the boiler buffer isn't that the highest possible temperature with the rest of the tank progressivly lower as you go down?

hr

 

[solarguy]

If you guys are dropping a septic tank in a hole, slinky a couple of rolls of pex instead of the copper. Do so going into it knowing that the heat transfer from pex will not be like copper but it's a hell of a lot cheaper. You might also consider having someone foam the outside of the tank to help cut down on the heatloss.
We played around with this idea a couple of years but never found anyone willing to gamble on it.

 

[Bob Rohr]

If you guys are dropping a septic tank in a hole, slinky a couple of rolls of pex instead of the copper. Do so going into it knowing that the heat transfer from pex will not be like copper but it's a hell of a lot cheaper. You might also consider having someone foam the outside of the tank to help cut down on the heatloss.
We played around with this idea a couple of years but never found anyone willing to gamble on it.

s-guy
I tried pex a number of years ago to replace some copper coils in an old Chiles Power System tank (pre Heatway days system). Didn't work out so well. An engineer at Watts Radiant later told me 3- 4 times as much pex would be needed to equal the heat transfer of copper. My experiment proved him to be correct with that calculation.

Perhaps pex with a bunch of that UltraFin product fastened to it?

hr

 

[Jersey Bill]

standard domestic water heaters use stratification in a similar way. As the hot water is drawn from the tank, the cold water fills in from the bottom up. this gives hot water at the outlet for much longer. If the water in the tank was mixed, the outlet water temperature would start to drop right away as the cold water averages with the hot water.

In a similar way a large storage tank with a vertical heat exchanger coil will be able to deliver, say, 130 degree water even though the bottom half (?) of the tank is well below 130. basically it allows a better drawdown and increased delta t of the storage.

Another benefit is that if there is a layer of cold water on the bottom of the tank, there will be less heat loss from the bottom because the temperature difference is less.

They tell me a typical 50 gallon water heater set at 130F with 55F incoming water will only proveid about 38-40 gallons of hot or warm water due to the mixing. I wonder hw hot the top of the tank is if the aquastat trips off at 130?

These days I see most homes with DHW recirc pumps. I'd guess the tank would be blended well at 130F?

Now if the tank was blended at 180F it seems there are more btu then in a tank with a stratification of cool water at the bottom.

I need to ring up some folks in the water heater industry to get cler on the math to determine which would store more energy. if the boiler or water heater is setpoint-ed at 130 or 180 for the boiler buffer isn't that the highest possible temperature with the rest of the tank progressivly lower as you go down?

hr

Correct. A typical storage heater will dump 80% of its water at the stored temperature before getting into the mixed water.
It is possible and done (like on nofossil's system) where the water is stored very high, then mixed down to use. This will store plenty more btu's but acording to code it has to be mixed down to prevent scalding. Also, if there is a mixing valve like that, you can't recirculate from past the mixing valve.
In a typical gas water heater, the temperature is measured near the bottom where the cold water fills from. If the bottom of the tank is at 130, then the top is close. On an electric water heater there are 2 elements high and low, each with their own thermostat but the lower is the lead, bur the upper has priority. When the upper calls for heat, it locks out the lower when it comes on.

Solarguy, since you brought it up, the current plan is to put a pre-cast 1000 gal, square, storage tank under a slab in the house stairhall at the ground level. Its only a few feet from the boiler room. We will insulate the tank with 4" of foam. Any heat lost up will go into the house nicely. There is a closet below the stairs where the corner of the tank will be accessable. We will cut a hole in the corner of the concrete cover big enough to get the coils in.
We thought about PEX, but by the time we figured out how to sink 3 times the linear footage at 1/3 the price per ft., we might as well go for the copper heat-x. The cost of 3/4" soft copper now is $3.35/lft, which is about $400 each for 120 ft, compaired to $700 from a vendor already formed.

Right now I am on the fence as for what do do about the controller. The tekmar boiler control is about $700, plus 2 more setpoint controllers for the storage tank control. I could put in a PLC and a small color touchscreen for as much money. The problem is that I would then have to write the ladder logic program to make it work. There is an economy of scale. I could also add all the controls for the wood boiler into the same unit, so some money would be saved there.

 

[Jersey Bill]

As the design matures, things change.
I noticed that the primary loop, the storage, and the domestic hot water all require about 15 gpm. Previously there were 3 pumps doing the work here, so I changed the DHW and the storage so that there are 3-way diverting valves (2 position actuator) , and put a little more power in the primary pump, thus eliminating 2 pumps.
The up front cost is about the same (I am working on getting pricing), but it will use less power and I believe that the valves will be more reliable than the pumps.

As for the controls, its been tough. A tekmar boiler control (w/ 2 reset loops) is about $700. Their storage tank control is about the same price, and its really for solar and overkill for my application. I came up with a scheme to use 2 or 3 setpoint controls, plus some control relays for the storage tank control, and if I use electronic controllers (for a good temp display) its not much cheaper.

For the same $1,400, I am going to get a Siemens S7-200 PLC and a 6" color Maples touch screen. The plc will be set up with 8 thermocouple inputs, 16 digital inputs, and 26- 2 amp relay outputs. The controller will handle inputs from the t-stats, all the zone valves, pumps, and mixing & diverting valves.
I still need a few control relays for the pumps, a 24 vac power supply, 2 fuses, terminal blocks, and an enclosure to put all the stuff in.

It wont be cheap, but it will be flexible enough to do anything that needs to be controlled in the boiler system.

 

[sled_mack]

HR,

I think stratification helps if you are not saturating your storage tank. I only really saturate my tank to full heat right before going on vacation. Even then, I'm heating from the top of the coils to the bottom, but as the top of the tank gets to the temp of the water coming out of the boiler, the heat gets pushed further down in the tank. The advantage is that the coils for the house have the water flowing bottom to top. This allows the water going to the house to get as close to the storage tank temp as possible.

It definitely requires more wood for me to saturate the tank to full temp. Normally, I can run the tank between 170 and 180 (measured dead center) with no extra effort on the part of the boiler and plenty of heat for the house. It takes a lot more wood to push the center of the tank to 200 deg. Granted, I can shut down longer once it is saturated to that temp, but I still think it uses more wood that way, so I don't do that for normal operation.

My top and bottom thermometers have died, so I don't really know how much stratification I have now. They actually died shortly after install, so I never got a good feel for it. And it hasn't been a priority for me to replace them, either.

 

[sparke]

sled_mack, What are you using for a storage tank?

 

[sled_mack]

It's an FRP (Fiberglass Reinforced Plastic) tank. It was used as a holding tank for hydraulic fluid for a large printing press. The hydraulic fluid must have come back hot, because it was rated for something like 300 deg F.

It was cheap ($300) off eBay, and works like a champ for me. It's 10 ft across and about 5 ft deep.

 

[sparke]

Thanks for the info. You running pressurized with dip tubes?

 

[sled_mack]

Boiler is pressurized with copper coils in the tank. Is that what you meant by dip tubes?

Pic below.

 

[sparke]

No I took it for granted you pressurized the tank and did not use internal Hex. The pic answers all my questions. Thanks!